AVS1996 Session TF+SS-MoA: Thin Film Sensors

Monday, October 14, 1996 1:30 PM in Room 201B

Monday Afternoon

Start	Invited?	Item
1:30 PM	Invited	TF+SS-MoA-1 Thin Film Challenges and Opportunities in the Development of Gas Microsensor Arrays S. Semancik, R. Cavicchi, F. DiMeo, Jr., J. Suehle, N. Tea (National Institute of Standards & Technology) Various detection principles and materials can be employed in constructing solid state gas sensing devices. Whatever the approach, practical fabrication and performance (selectivity, speed, stability) issues must be addressed that relate directly to the form and properties of the transducing material(s). In this presentation, we describe a range of thin film microfabrication challenges associated with bringing existing and new concepts in chemical sensing into a reliable, manufacturable planar technology. The discussion is centered on conductometric, "microhotplate" arrays being developed for application-specific, gas mixture analyses. Each suspended microhotplate in an array (realized by Si micromachining) serves as a ~100 x 100 \mu\m\super 2\ microsensor platform, with individually-addressable, fast (~ 5 ms) temperature control between 20 and ~ 800oC, and 4 top-surface contacts for measuring film conductances. Thin (~50-200 nm) films of pure and modified semiconducting oxides (such as SnO\sub 2\, TiO\sub 2\, ZnO and WO\sub 3\) are deposited over the contacts as partially-selective, active components. Gas-induced conductance changes of multiple films are employed as the basis for detection and quantification. Specific thin film topics to be discussed will include: CVD self-lithographic methods for local deposition of oxide and metal films on the microhotplates; stoichiometry, crystallinity and thickness effects on sensing response; oxide film tailoring, by dopants, and surface-dispersion of monolayer regime catalytic additives; and, oxide film-metal contact interfaces. Opportunities for highly-efficient surveys on film process/property relationships that have evolved from our array work will also be described, as will studies of heteroepitaxial/ultrathin (1-10 nm) oxide films that hold potential for new sensing structures and detection schemes.
2:10 PM		TF+SS-MoA-3 Controlled Growth of WO\sub 3\ Gas Sensing Thin Films L. LaGore, J. Paulus, D. Frankel, R. Lad (University of Maine) WO\sub 3\ films containing catalytic metal additives such as Au and Ru can be used as gas sensing elements in Taguchi-type sensors or in highly sensitive fiber optic or surface acoustic wave microsensors. The microstructure and morphology of the WO\sub 3\ film as well as the concentration and distribution of the catalytic metal additive have a large influence on the sensitivity, selectivity and stability of the sensor. We have used reactive rf magnetron sputtering and e-beam evaporation of WO\sub 3\ to grow WO\sub 3\ films which exhibit a range of microstructures on r-cut sapphire (\alpha\Al\sub 2\O\sub 3\) and y-cut quartz (\alpha\-SiO\sub 2\) single crystal substrates. The effect of the presence of an ECR oxygen plasma during film growth has also been studied. On both substrates, the films are amorphous when grown at temperatures below 400\super o\C. Post-deposition annealing in air or O\sub 2\ induces the formation of a polycrystalline microstructure. In situ crystallization of the WO\sub 3\ films can be achieved at deposition temperatures above 400\super o\C. RHEED observations during growth indicate that WO\sub 3\ grows epitaxially on r-cut sapphire. High temperature growth on quartz yields a polycrystalline film. AFM analysis has been used to compare the grain size and roughness of the various films. The use of an ECR Ar/O\sub 2\ plasma during sputtering or evaporation appears to increase the crystalline quality of the WO\sub 3\ films presumably due to enhanced surface diffusion during growth. The correlation between the type of film microstructure and the response to H\sub 2\, NO\sub x\, and NH\sub 3\ gases will discussed.
2:30 PM		TF+SS-MoA-4 In Situ Resistance Characterization of Oxide Thin Film Growth Phenomena F. DiMeo, Jr., R. Cavicchi, S. Semancik, J. Suehle, N. Tea (National Institute of Standards & Technology); J. Kelliher (Microelectronics Research Lab) We have developed a new approach for the in situ electrical characterization of growing oxide layers. This method uses the transient resistance response on a millisecond time scale to directly probe such phenomena as the onset of nucleation and growth, the effects of variations in the deposition environment, and the effects of rapid thermal processing. This technique is based on a microhotplate structure with extremely fast heating characteristics, 10^6 degrees/s, and which also has surface electrical contacts which permit conductance measurements of growing films. We have used this micromachined structure in conjunction with MOCVD to investigate the deposition of conducting oxides such as SnO\sub 2\ and ZnO for gas sensing applications. Oxide thin films have been deposited at temperatures between 300 to 500 =B0C= , and effective growth rates are approximately 3 nm/s. This can be reduced by a factor of 5 to 10 by pulsed temperature processing. This involves rapidly turning the heater on and off at a reduced duty cycle, thus reducing the amount of growth that occurs. Film resistivities in the range of 10^5 to 10^8 ohm/cm^2 have been measured as a function of deposition time and indicate that the very early stages of growth proceeds in a similar fashion for both materials. Oscillations in the resistance of growing ZnO thin films have been observed and attributed to variations in the precursor concentration. The films are, in essence, gas sensing the environment they are growing in. Finally, implications of how this in situ measurement technique can be used to monitor properties relevant to gas sensing, e.g. morphology, grain size, and oxygen stoichiometry, will be discussed.
2:50 PM	Invited	TF+SS-MoA-5 Reduced-Coordination Cation Sites at Surface Oxygen Vacancies on SnO\sub 2\(110) D. Cox (Virginia Polytechnic Institute & State University) Much of the reactivity of oxide surfaces is attributed to the presence of surface defects. Our work over the last few years has concentrated on investigating the peculiarities of metal oxide surface chemistry associated with surface oxygen vacancies. The research effort is focused primarily on the SnO\sub 2\(110) surface because of the flexibility it allows in the formation (i.e., intentional introduction) of two different types of surface oxygen vacancies using combinations of high pressure oxidation, vacuum annealing and ion bombardment. It has been found that the dissociation of weak Bronsted acids occurs at specific surface sites associated with oxygen vacancies. A chemical characterization of the acid/base properties of oxygen vacancies on SnO\sub 2\(110) will be reported. NH\sub 3\ thermal desorption studies have shown that the Lewis acidity of the surface varies with preparation. Five-coordinate Sn\super 4+\ cations are the weakest Lewis acid sites, while four-coordinate Sn\super 2+\ cations associated with a particular type of oxygen vacancy are the strongest Lewis acid sites (i.e., they give the highest heat of adsorption of ammonia.) Cations with the lowest coordination numbers have intermediate acidities. Some preliminary results of computational and experimental investigations of the geometric and electronic structure of oxygen-deficient SnO\sub 2\(110) surfaces will also be presented.
3:50 PM		TF+SS-MoA-8 Interfacial and Electronic Properties of SiC-based Schottky Diode Gas Sensors Annealed at 425 C L. Chen, G. Hunter, P. Neudeck, G. Bansal (NASA Lewis Research Center); J. Petit (NYMA-NASA Lewis Research Center); D. Knight (Cortez III-NASA Lewis Research Center) Silicon Carbide-based (SiC) Schottky diode gas sensors using palladium (Pd) as the catalytic metal have been shown to detect hydrogen and hydrocarbons at elevated temperatures with high sensitivity. In this work, long-term annealing at 425 C in air up to 140 hours is carried out on SiC-based Schottky diode gas sensors to determine the effect of extended heat treating on the diode properties and gas sensitivity. Two different Schottky diode structures are examined: 1) 400 A Pd deposited directly onto 6H-SiC (Pd-SiC), and 2) 400 A Pd deposited onto approximately 50 A of silicon dioxide (SiO2) thermally grown on 6H-SiC (Pd-SiO2-SiC). The capacitive and current carrying characteristics of the diodes were measured before and during the annealing. The surface and interface of the diodes were then characterized using Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectrometry (EDS), and Auger Electron Spectroscopy (AES). The annealing changed both sensors' electronic properties and sensitivity to hydrogen with the effect being more pronounced in the Pd-SiO2-SiC sample. SEM and EDS studies of the Pd surface for both samples after annealing show cluster formation and background regions with grain structure observed in both regions. AES of the Pd surface suggests that the annealing resulted in silicon oxide and palladium silicide formation. AES depth profiles reveals that the annealing broadened the interface region with significant palladium silicide formation throughout the interface. This work shows that for long-term, high temperature applications stabilization of the Schottky diode structure is necessary.
4:10 PM		TF+SS-MoA-9 Characterization of the Resistance Change Produced by H\sub 2\, H\sub 2\S, CO, and O\sub 2\ Adsorption onto Iron Films M. Shanabarger (University of California, Santa Barbara) Measurements have been made of the change in electrical resistance which results from the chemisorption of H\sub 2\, H\sub 2\S, CO, or O\sub 2\ onto the exposed surface of an evaporated Fe film. The thickness dependence of the resistance change for all the indicated species was determined at substrate temperatures of about 300 K. The temperature dependence of the resistance change was determined for H\sub 2\, CO, and O\sub 2\ for fixed film thicknesses. In addition, the magnitude of the resistance change at about 300 K for H\sub 2\S and O\sub 2\ was correlated with the amplitude of the adsorbing species Auger electron spectra during adsorption. These measurements are compared with various models to establish the origin of the resistance change. The data is most consistent with a model in which the resistance change results from a change in the conduction electron density of states.\super *\ Work supported by NASA-Ames Cooperative Agreement, NCC-2-63.
4:30 PM		TF+SS-MoA-10 Formate Adsorption on Epitaxial Cu(100) Films: Broadband Reflectance and dc Resistance E. Krastev, D. Kuhl (Michigan State University); R. Tobin (Tufts University) Adsorption of many gases, including O, CO and H on Cu reduces the broadband infrared reflectance of the surface by ~1%. This effect has been convincingly attributed to diffuse scattering of conduction electrons by the adsorbates. We have simultaneously measured both the reflectance change and the dc resistance change induced by adsorbed formate and O on epitaxial Cu(100) films. For O the two changes are proportional, as predicted by the scattering model. For formate, however, a considerable resistance change is accompanied by essentially zero (<0.1%) reflectance change. This result is incompatible with the scattering mechanism. Effects of formate coverage, substrate temperature and film thickness, vibrational spectra and possible explanations will be presented. The films, 50-100 nm thick, are grown by thermal evaporation on H-terminated Si substrates. They are epitaxial and display distinct LEED spots, despite rms surface roughness of ~1 nm. Contamination levels are less than 5%.